In an image display system for changing a display of image data based on a visual field changing instruction, one or a plurality of visual field changing start region associated with a visual field changing operation, which defines display update processing contents of the image data, is displayed in a display window. When a pointer enters the visual field changing start region, the visual field changing operation associated with the visual field changing start region is activated, and the display of the image data is updated in accordance with the display update processing contents defined by the activated visual field changing operation, based on a relative movement amount of the pointer based on the position of the pointer at a time when the visual field changing operation has been activated. In the case where the pointer is moved to outside of the visual field changing start region corresponding to the activated visual field changing operation and is placed in the display window, the visual field changing operation is deactivated.